Glasshouse and laboratory studies were undertaken to evaluate a range of extractants for determining the boron status of a suite of predominantly acidic soils. In addition to hot water (commonly used as an extractant for soil boron), hot 0.01 M CaCl,, 0.05 M mannitol/O.Ol M CaCI,, 0.05 M mannitol (buffered at pH 7 . 9 , 1 M NH,OAc, 5% ( v h ) glycerol and 0.01 M tartaric acid were assessed as extractants since these reagents can complex boron.Sunflower (Helianthus annuus cv. Hysun 31) was grown with three levels of applied boron (0, 0.5 and 1.0 kg B ha-') in each of 20 soils. Relative dry matter yield, tissue boron concentration, boron uptake and relative boron uptake of control plants (0 kg B hal) were determined. Both hot water and hot 0.01 M CaClz extractable boron were related to each of these plant parameters, whereas boron extracted by the other procedures was not.Levels of boron extracted with mannitol and glycerol were very low in comparison to those displaced from the soil by the refluxing procedures. The inability of mannitol to extract boron from these soils is discussed in relation to the chemistry of mannitol-boron complexes, and it is suggested that mannitol would not be an effective extractant for boron in acid soils.Soil solution extracts (obtained after equilibration at 10 kPa matric suction) had concentrations < 0.1 ~g B mll . Because of the low concentrations and narrow range across the 20 soils, soil solution boron was not related to plant growth parameters obtained from the glasshouse study.

The phosphorus status of each of 26 surface soils from Queensland was characterized by laboratory measurements and a glasshouse experiment. The glasshouse trial investigated the response between applied P in each soil and maize (Zea mays) dry matter yield. In the laboratory, the quantity of soil P was estimated by extraction with 0.5 M NaHCO3 (PB), and the intensity was estimated by soil solution P, 0.005 M CaCl2 extraction and equilibrium phosphorus concentration (EPC). Phosphorus-sorption curves were established for each soil and the data were used to derive the buffering index (BI) and equilibrium buffer capacity (EBC). Four single-point sorption indices were also determined. The desorption buffer capacity (dBC) of each soil was obtained in the laboratory by equilibrating soil samples with anion exchange resin for periods ranging from 0.1 to 18 h. This paper reports the relationships between the various P parameters and (i) the P uptake by maize (Zea mays) grown in untreated soil, and (ii) the amount of added P required for 90% maximum yield. Intensity, as estimated by EPC, was significantly (P < 0.001) correlated with P uptake. Any of the BI, EBC or the single-point sorption indices significantly improved the variation in P uptake accounted for by PB alone, but not to the same level as that obtained with EPC alone. When PB was combined with dBC, more variance was accounted for in P uptake than by using any of the adsorption buffer capacity measurements. The effects of quantity, intensity and buffer capacity on P availability are discussed in terms of their effects on P diffusion. For the suite of soils studied, it is concluded that intensity is the prime factor governing availability, and that the usefulness of adsorption buffer capacity measurements depends on their correlation with desorption buffer capacity. Variation in P requirement was best described by a combination of EPC and the Mitscherlich curvature coefficient, or EPC and one of the single-point sorption indices. As the single-point sorption indices were highly correlated with desorption buffer capacity, adsorption buffer capacity, and the curvature coefficient, they offer a convenient measure of the sorption properties of a soil.

Negative potassium (K) balances in all broadacre grain cropping systems in northern Australia are resulting in a decline in the plant-available reserves of K and necessitating a closer examination of strategies to detect and respond to developing K deficiency in clay soils. Grain growers on the Red Ferrosol soils have increasingly encountered K deficiency over the last 10 years due to lower available K reserves in these soils in their native condition. However, the problem is now increasingly evident on the medium-heavy clay soils (Black and Grey Vertosols) and is made more complicated by the widespread adoption of direct drill cropping systems and the resulting strong stratification of available K reserves in the top 0.05-0.

Seedling macadamia (Macadamia integrifolia cv. Hinde) were grown in pots in two glasshouse experiments for 23 weeks. Experiment 1 comprised ten soils at two P levels (nil and a rate calculated to be non-limiting to growth) with six replications. Experiment 2 consisted of another two soils with eight rates of added P (0-2560 mg P per 4 L pot) and six replications. Whole plant tops were harvested, dried and weighed, and leaves analysed for P. In addition, leaves from Experiment 2 were analysed for Cu, Zn, Mn and Fe. Roots were recovered from the soils, separated into proteoid and non-proteoid root material, dried and weighed. Control (nil added P) soils were analysed for soil solution P and Colwell, Olsen, Bray 1 and 0.005 M CaCl2 extractable P. At 90% of maximum whole plant top growth, P concentration in the leaf was 0.08%. When the leaf Fe/P ratio < 0.07 in Experiment 2, there was a significant yield depression associated with symptoms of severe iron chlorosis. Critical soil P levels at 90% of maximum whole plant top growth were 50, 23 and 29 mg kg-1 for Colwell, Olsen and Bray 1 extractable P, respectively. It was not possible to define a critical CaCl2 extractable P or soil solution P concentration because of the large increase in relative growth with a small increase in these parameters. Proteoid root growth (as a percentage of total root weight) decreased with increasing level of soil phosphorus, and there were very few proteoid roots at >100 mg kg-1 Colwell extractable P. Applying P to maintain high soil test levels (>100 mg kg-1 Colwell extractable P) would have detrimental effects on proteoid root development.

Hedley et al. (1982) developed what has become the most widely used (and modified), phosphorus (P) fractionation technique. It consists of sequential extraction of increasingly less phytoavailable P pools. Extracts are centrifuged at up to 25000 g (RCF) and filtered to 0.45 µm to ensure that soil is not lost between extractions. In attempting to transfer this method to laboratories with limited facilities, it was considered that access to high-speed centrifuges, and the cost of frequent filtration may prevent adoption of this P fractionation technique. The modified method presented here was developed to simplify methodology, reduce cost, and therefore increase accessibility of P fractionation technology. It provides quantitative recovery of soil between extractions, using low speed centrifugation without filtration. This is achieved by increasing the ionic strength of dilute extracts, through the addition of NaCl, to flocculate clay particles. Addition of NaCl does not change the amount of P extracted. Flocculation with low speed centrifugation produced extracts comparable with those having undergone filtration (0.025 µm). A malachite green colorimetric method was adopted for inorganic P determination, as this simple manual method provides high sensitivity with negligible interference from other anions. This approach can also be used for total P following digestion, alternatively non-discriminatory methods, such as inductively coupled plasma atomic emission spectroscopy, may be employed.